The present disclosure relates generally to power supply compensation, and more particularly to power supply parameter adjustment to improve light load efficiency.
Light load efficiency of inductor-based power converters is improved through operation in a discontinuous or a transition conduction mode through use of a diode emulation circuit. Referring to FIG. 1, a power stage 100 of a switching power supply is illustrated. Power stage 100 can be operated in discontinuous or transition conduction mode, in which current through an inductor 104 is prevented from becoming negative. A control MOSFET 101 charges inductor 104 when on. When a rectifying MOSFET 102 is turned on, inductor 104 discharges, and current flowing through inductor 104 decreases toward zero. When the current through inductor 104 reaches zero, MOSFET 102 is turned off, thereby preventing current in inductor 104 from becoming negative.
The efficiency obtained by operating power stage 100 in discontinuous or transition conduction mode during light-load operation can be diminished if MOSFET 102 is not turned off when inductor 104 has zero current flowing. For example, if MOSFET 102 is turned off before the current in inductor 104 reaches zero, a body diode of MOSFET 102 conducts, leading to conduction losses that can negatively impact efficiency. If MOSFET 102 is turned off after the current in inductor 104 has passed zero to become negative, the voltage at switching node SW increases sharply to the input voltage level and MOSFET 102 experiences switching losses that negatively impact efficiency.
Accordingly, switching MOSFET 102 off with appropriate timing to permit the current in inductor 104 to reach zero and avoid becoming negative is an important aspect for control of power stage 100. One technique for detecting zero current in inductor 104 involves measuring a voltage across sense resistor Rs. When the voltage across sense resistor Rs becomes zero, as measured by comparator 106 in control 108, an output of comparator 106 changes state. When the voltage of sense resistor Rs becomes zero, the change in state of the output of comparator 106 permits control circuit 108 to supply a signal to turn off MOSFET 102. Ideally, MOSFET 102 is turned off when the current through inductor 104 becomes zero, to limit conduction and switching losses in power stage 100. Zero current in inductor 104 corresponds to zero measured voltage across sense resistor Rs.
However, in practice, comparator 106 has some offset voltage that can deviate during circuit operation. The offset voltage may cause comparator 106 to change state before or after the voltage on sense resistor Rs reaches zero. Thus, the offset voltage and deviations in the value of the offset voltage can degrade circuit performance due to increased conduction and switching losses, as mentioned above.
In addition, control circuit 108, including comparator 106 and a MOSFET driver (not shown), includes some delay in propagating a signal to turn off MOSFET 102. The delay can cause inaccurate timing for turning off MOSFET 102.
Furthermore, sense resistor Rs is specified to have a small resistance to improve heavy load efficiency. During light-load operation, the resistance value of sense resistor Rs has a greater impact on the operation of power stage 100.
One solution to overcome the above challenges is to provide a compensation voltage to comparator 106 to cancel the effects of the offset voltage. Such a compensation voltage can also compensate for a delay in control circuit 108 to improve timing. However, the compensation voltage is typically set once in practice, on a case-by-case basis for each power stage 100, such as may be provided during manufacture. It is difficult to set the compensation voltage accurately with respect to the value of sense resistor Rs to avoid negatively impacting light-load efficiency. In addition, an optimal value for the compensation voltage is dynamic, and varies as a function of external conditions, such as an output voltage and/or inductor characteristics related to temperature or operating parameters, for example. A static compensation voltage is unable to adequately compensate for the dynamic conditions present in power stage 100 under normal operating conditions. Accordingly, during normal operation, power stage 100 experiences a loss in efficiency due to variations in the offset voltage inherent to comparator 106 and variable signaling delays in control circuit 108, which prevent MOSFET 102 from being turned off when inductor 104 has zero current.